Back to the main menu


GUIDING (JTR March 2012) CONTENTS
  1. INTRODUCTION
  2. IR GUIDING USING SPEX GUIDER/SLIT-VIEWER
  3. VISIBLE GUIDING USING THE OFF-AXIS GUIDER
  4. VISIBLE GUIDING USING MORIS

  1. INTRODUCTION
    
To maximise throughput of the spectrograph it is important
to keep the science object in the slit. Since SpeX is
optimised to use slit widths about the same size as the
seeing it is necessary to use active guiding rather than
rely on telescope tracking alone.
    Back to the top


  2. IR GUIDING USING THE SPEX GUIDER/SLIT-VIEWERi (GUIDEDOG) 
    

   Guiding can be done on spill-over flux from 
   the object in the slit or on an object in the 
   60x60 arcsec field of the imager. In photometric conditions
   and in median seeing (0.7") the magnitude limit for auto-guiding 
   on a star in the slit is about J=15; the magnitude limit for 
   manual guiding (using the paddle) on a star in the slit
   is about J=18. The magnitude limit for auto-guiding
   on a star in the field is about J=18. These limits
   depend on seeing and slit width (no spillover in wide slits). 

   Auto-guiding also works very well on extended objects
   such as galaxy nuclei and small disks (eg. 4 arcsec diameter
   Uranus) so long as the objects are bright enough and
   guiding on the peak or centroid is acceptable. Positioning
   a slit at a particular location on Jupiter or Saturn, for
   example, must be done using non-sidereal telescope tracking
   with manual updates provided by observing the slit location
   in the SpeX slit-viewer. 

   For details read the document 'How to set up the IR guider'.
    Back to the top


  3. VISIBLE GUIDING USING THE OFF-AXIS GUIDER (SMOKEY) 
    

   Visible guiding can be done using the off-axis guider. The location
   of the off-axis guide fields is shown below by the rectangular green 
   boxes. The center of the boxes are located 244" east, 244" west, and
   293" south of center. The dimesions of the individul guide fields are
   100"x400", for a total off-axis field of about 30 sq. arcmin. The 
   off-axis guider should be used as a last resort since there is some 
   flexure between it and SpeX. However, it can be used for integrations 
   of up to 30 minutes before recentering of the target in the slit is
   recommended. The off-axis guider does not compensate for differential
   atmospheric refraction due to guiding in the optical while observing 
   in the NIR. However, visible guiding with MORIS does (see below).

	
    
	   
    
		   
    


   Magnitude limit     Itime
      (no moon)        (sec)
       V=10.0           0.7
       V=12.7           3.5
       V=14.0           6.5
       V=15.0           8.0

   Place the science object in the slit and ask the telescope
   operator to start guiding on a suitable off-axis star. 
   Make sure that the science object does not move out of the
   slit when guiding is started (it should not). Set up the
   telescope nod position as described in     
   'How to set up the the IR guider'.
    Back to the top


  4. VISIBLE GUIDING USING MORIS 
    

   Visible guiding can be done using MORIS. MORIS is a CCD system comprising
   re-imaging optics, filter wheel, and cooled CCD. It is mounted on the 
   side of SpeX and is fed by a dicroic in SpeX. There is a choice of two
   dichroics:
	
   1) 0.80 micron cut-on which sends wavelengths < 0.80 micron
      to the wavefront sensor (less light to guide on but
      spectral coverage > 0.8 micron)
   2) 0.95 micron cut-on which sends wavelengths < 0.95 micron
      to the wavefront sensor (more light to guide on but 
      less spectral coverage > 0.95 micron)

   The FOV of MORIS is about 80"x80".

   Guiding with MORIS is designed to be very similar to using Guidedog with guide stars
   in its FOV. Once the target is placed in the slit (beam A) draw a box around the guide 
   star in the MORIS FOV and select it in the MORIS DV display (click 'Ga'). Then start
   'Guiding' with MORIS. It maybe necessary to nudge the target star as viewed in Guidedog
   into the slit. This can be done either by moving the guide star with the paddle (the
   guide box will move with ther star) or by incrementally abjusting the guide box
   position with the arrows ('*GBOX CenXY'). The target can be nodded completely out of
   the slit by selecting a different B beam position (click 'Gb'), otherwise the default
   will be to nod within the slit.


    
   
    
	   
    

  A differential atmospheric refraction algorithm allow for accurate guiding in the NIR 
  while using an optical guide star. In photometric conditions and median seeing (0.7") 
  we have successfully guided with MORIS down to Sloan r'=20.4 and i'=19.9 using an 
  integration time of 15 sec.

  For more information see the     'MORIS documentation'.
	 



    Back to the top